We can't have a why, because the claim that the norm is L2 is axiomatic [1]. However, as the sibling comment notes, we can argue from consistency. Would any oth

We can't have a why, because the claim that the norm is L2 is axiomatic [1]. However, as the sibling comment notes, we can argue from consistency. Would any other norm give us a sensible theory? Or we can put quantum theory in a larger framework of theories and discuss why the L2 norm makes more sense within that framework. Scott Aaronson discusses some of these idea in his lecture [2]. [1] Feynamn on why https://www.youtube.com/watch?v=36GT2zI8lVA&pp=ygULZmV5bm1hbiB3aHk%3D https://www.youtube.com/watch?v=36GT2zI8lVA&pp=ygULZmV5bm1hb... [2] https://www.scottaaronson.com/democritus/lec9.html https://www.scottaaronson.com/democritus/lec9.html